Some studies show enhanced public safety. Others suggest the opposite. One study concluded that several dozen men a month are killed as a consequence of the laws. In a speech this week, U.S. Attorney General Eric Holder showed himself to be among the skeptics.

The resulting ambiguity, like the Zimmerman trial itself, in which he was acquitted of all charges in the death of 17-year-old Trayvon Martin, may confound anyone hoping for a neutral finding of fact. It also allows advocates and critics alike to cherry-pick their preferred experts, which can enliven debate but complicate lawmaking.

"It's good that whatever perspective you take is based on data and not just anecdotes," Miami-based attorney Leigh-Ann Buchanan said in an interview this week.

Buchanan is a co-chair of an American Bar Association task force that's studying stand-your-ground laws. The task force has held three hearings so far, and it plans another next month in San Francisco. The group will report next year on various questions surrounding the laws, including how they "may impact public safety," she said.

This week, Holder joined those who assert that the stand-your-ground laws make matters worse. Addressing the NAACP on Tuesday in Orlando, Fla., he declared that the laws may "contribute to more violence than they prevent."

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"By allowing and perhaps encouraging violent situations to escalate in public, such laws undermine public safety," the nation's top law enforcement officer said.

Starting with Florida in 2005, at least 24 states have adopted some variation of a stand-your-ground law, Buchanan said. Additional states have adopted similar policies through state court rulings. Their general thrust is to remove the long-standing legal "duty to retreat" in the face of danger while in a public place. Instead, individuals may defend themselves, with lethal force if necessary, so long as they are in the public place legally.

Last year, two Georgia State University economics researchers concluded that the laws are "associated with a significant increase in the number of homicides among whites, especially white males." In a 55-page paper published by the National Bureau of Economic Research, authors Chandler B. McClellan and Erdal Tekin estimated that "between 28 and 33 additional white males are killed each month" as a result of these laws.

"These laws are also associated with a significant increase in emergency-room visits and hospital discharges related to firearm-inflicted injuries," the Georgia State economists said.

Texas A&M researchers Mark Hoekstra and Cheng Cheng reached similar conclusions last year in a 43-page study that found a "statistically significant 8 percent net increase in the number of reported murders and non-negligent manslaughters" in states with the enhanced self-defense laws.

Private citizens committed 196 justifiable homicides nationwide in 2005, the year states began crafting stand-your-ground laws, according to the FBI's Uniform Crime Reporting system. The FBI defines justifiable homicide as "the killing of a felon during the commission of a felony." By 2011, the number of reported justifiable homicides jumped to 260. In Florida, that number has nearly tripled since the years immediately prior to passage of the self-defense law.

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Sanford police told to hold Zimmerman evidence

Police in central Florida say the U.S. Department of Justice has placed a hold on all evidence related to the fatal shooting of 17-year-old Trayvon Martin by former neighborhood watch volunteer George Zimmerman.

Sanford Police Capt. James McAuliffe confirmed the hold Thursday. That includes the gun Zimmerman used to shoot Martin, which Zimmerman would otherwise be legally entitled to reclaim.

Zimmerman was acquitted over the weekend of charges in Martin's death last year.

Trayvon Martin's parents "stunned absolutely" by verdict

For the first time since George Zimmerman was acquitted of murdering their son, Trayvon Martin's parents gave TV interviews Thursday to decry the verdict and ask the federal government to continue to investigate.

Sybrina Fulton, Martin's mother, said in a "CBS This Morning" interview that she was "stunned absolutely" by the not-guilty verdict handed down Saturday by the six-woman jury in Seminole County, Fla.

"I thought surely that he (Zimmerman) would be found guilty of second-degree murder, manslaughter at the least," Fulton said.

"My son was unarmed, and the person that shot and killed him got away with murder," Fulton told CNN's Anderson Cooper, as she made the case for a change in self-defense laws.